Fractionation control system



April 9, 1963 L. D. KLEISS Filed Sept. 4, 1959 FRACTIONATION CONTROLSYSTEM FLOW TRANSDUCER 'I I I l I "1 I (I+KAT)2 2o 27 I TRANSDUCER I I3| 23 I I I I I I I 6 J :%I o 2 35 1 l 26 as:

I y l 2 s l 2 I I n 3 I I T FLOW RATIO 6! 0 FEED TRANSDUCER CONTROLLER I2 j OR COMPUTER J o 30 L F 62 2 I3 9 I- U 1 E 1 E 1 -L I STEAM l l5 |8INVENTOR L.D.KLEI$S A 7' TORNEVS United States Patent 3,085,050FRAQTEGNATION CONTRQL SYSTEM Louis D. Kleiss, Burger, Tern, assignor toPhillips Petrcleum Company, a corporation or Delaware Filed Sept. 4,1959, Ser. No. 833,265 2 Claims. (Cl. 202-160) This invention relates toimproved systems for controlling the operation of fractionation columns.

In recent years increasing use has been made of fan coolers forcondensing overhead vapor from fractionation columns. However, this typeof cooler has resulted in a rather serious operating problem because itis dithcult to control the exact amount of cooling provided. Suchschemes as fan speed control, variable pitch fan blade control and hotvapor by-pass control have been employed in an attempt to solve thisproblem, but have not been entirely satisfactory. Sudden atmospheretemperature changes, such as occur during a rainstorm, for example,result in condensation of vapor within the column because of cooling ofthe reflux below its bubble point. This results in an increase inoverhead product purity at the expense of decreased overhead productrate, and therefore, a decrease in column throughput.

it has recently been discovered that the control of fractionationcolumns can be improved by computing the amount of internal reflux inthe column and utilizing this computation for control purposes. Internalreflux is defined herein as the external reflux returned to the columnplus .the vapor which is condensed near the top of the column by thesub-cooled external reflux. Such a computation can be made from ameasurement of the rate of flow of the external reflux and a measurementof the temperature differential between external reflux returned to thecolumn and a region near the top of the column, such as the overheadvapor. Signals representative of these two measurements are combined soas to provide a measurement of the internal reflux in the column. Inaccordance with the present invention, improved fractionation columncontrol systems are provided which are based upon measurementsrepresentative of internal reflux, but which do not actually require acomputation of the internal reflux. Fractionation column control systemsare provided wherein the ratio of internal reflux to overhead vapor flowis maintained at a preselected value. These control systems result inincreased product purity with a smaller expenditure of energy.

Accordingly, it is an object of this invention to provide improvedfractionation column control systems which are based on measurementsrepresentative of internal reflux in the column.

A further object is to provide a fractionation control system whereinthe ratio of internal reflux to overhead vapor flow is maintained at apreselected value.

Other objects, advantages and features of this invention should becomeapparent from the following detailed description, in conjunction withthe accompanying drawing in which:

Accompanying drawing which is a schematic representation of oneembodiment of the control system of this invention.

Referring now to the drawing in detail there is shown a conventionalfractionation column 10 which is provided with a number of vapor-liquidcontactingtrays. A fluid mixture to be separated is introduced intocolumn 19 through a conduit 11. Steam, or other heating medium, iscirculated throughthe lower region of column 10 by a conduit 12 at apredetermined rate which is maintained by a flowcontroller 13 thatadjusts a valve 14. A kettle product stream is withdrawn from the bottomof column 10 through a conduit 15. The rate of product withdrawal3,085,059 Patented Apr. 9, 1963 through conduit 15 is regulated by aliquid level controller 17 which adjusts a valve 18. Vapors arewithdrawn from the top of column 10 through a conduit 19 whichcommunicates with an accumulator 2t) through a condenser 21. The controlsystem of this invention is particularly useful when condenser 21 is afan cooler, but the invention is by no means restricted to such acondenser. Condensed liquid in accumulator 20 is removed through aconduit 22 which communicates with the inlet of a pump 23. Pump 23returns a portion of this liquid to the upper region of column 10through a conduit 24. This flow of liquid through conduit 24 is referredto hereinafter as the external reflux. The remainder of the liquidremoved from accumulator 20 constitutes the overhead product and isremoved through a conduit 26. The flow through conduit 26 is regulatedby a liquid level controller 27 which adjusts a valve 28 to maintain apredetermined liquid level in accumulator 20.

As previously mentioned, the control system of this invention is basedupon a measurement representative of the internal reflux in the column,even though this internal reflux is not measured direclty. In order toexplain the operation of the controller of this invention, an equationwhich is representative of the internal reflux in a fractionation columnwill be derived.

The material balance at the top tray of the fractionator can beexpressed:

e ed" i l ii o where:

h enthalpy of external reflux h =enthalpy of internal reflux H :enthalpyof vapor streams (assumed to be equal) The enthalpy of the vapor streamsentering and leaving the top tray can be expressed:

where is the heat of vaporization of liquid on the tray. The enthalpy ofthe external reflux can be expressed:

h =h -C AT where:

C =specific heat of the external reflux stream AT=the diiference intemperature between the top tray and external reflux Equation 3 can besubstituted into Equation 2 to eliminate H and rewritten:

l( i+ o( i+ l 1 e e Equation. 4 can be substituted into Equation 5 toeliminate h and rewritten:

( l+ i V0) 1( l e) e p (6) From Equation 1 it is knowni o 1' e (7)Equation 7 can be substituted into Equation 6 and reducedto obtain:

3 Equation 8 can be modified by substituting a constant K for andsquaring both sides to obtain:

R, =R (1+KAT) 9 The term R in Equation 9 is measured by a flowtransducer 30 which establishes an output signal representative of theflow of external reflux through conduit 24. Such a signal canconveniently be established by a differential pressure transmitterconnected across an orifice in conduit 24. This pressure ditferential isrepresentative of the square of the flow. The term (Ll-KAT) isestablished by comparing the temperature of the reflux in conduit 24with the temperature of the vapor removed from column 10 through conduit19. A temperature transducer 31 is calibrated to provide an outputsignal representative of the term .(l-i-ATP when provided with inputsignals from respective sensing means, such as thermocouples. The termsC and 7x, and thus K, are substantially constant for any given fluidmixture. From this information, it is possible to provide a signalrepresentative of the internal reflux in the fractionation column.

The control system of the drawing is provided to maintain the ratio ofthe internal reflux to the overhead vapor withdrawal rate substantiallyconstant at a preselected value. The feed through conduit $1 1 ismaintained at a predetermined rate by a flow controller 60 whichregulates a valve 61. A second flow transducer 63 establishes an outputsignal which is representative of the square of the flow rate ofoverhead vapor through conduit 19. The output signals from flowtransducers 30 and 63 are applied to the respective inputs of a ratiocontroller or computer 62. The output signal from temperature transducer3-1 is applied to an input of controller 62 to adjust the set pointthereof. The output signal of controller 62 is representative of and isutilized to adjust a valve 35 in conduit 24 to control the flow ofexternal reflux to column 10.

The preselected value of the ratio of internal reflux to overhead vaporwithdrawal rate or where 0 represents the rate of withdrawal of overheadvapors through conduit 19. The signal applied to ratio controller 62from flow transducer 30 is representative of the term R and the signalapplied to ratio controller 62 from flow transducer 63 is representativeof the term 0 Temperature transducer 31 is calibrated by adjusting thezero point and the span so that the output signal, which is applied tothe set point of ratio controller 62, is representative of the term(1+KAT) The output of this transducer is reasonably linear over therange of AT normally encountered in column separation. The normal rangeof AT of the specific column under control is considered in making thecalibration. If greater accuracy is required, the transducer can becalibrated to yield the output term 1+KAT, and this signal can besquared using, for example, a Sorteberg Force Bridge to yield the term(1+KAT) This results in the signal being multiplied by the factor(l-i-KAT) by ratio controller 62. Controller 62 adjusts the flow ofexternal reflux through valve 35 to maintain the ratio of R to O at apreselected value. For any given value, this ratio is representative ofthe ratio of R to O. The particular ratio desired will depend on anumber of factors such as compositions, flow of feed, desired separationand size of the fractionation column, for example.

The control apparatus illustrated in the drawing can advantageouslyemploy conventional pneumatic or electrical equipment which is wellknown to those skilled in the art and which can be obtainedcommercially. Instead of a ratio controller, controller 62 can be acomputer, such as a force bridge of the type described in Us. Patent2,643,055, for example, and can be provided with input signals R(l-i-KAT) and 0 The output signal of the computer is representative ofwhich, as indicated by the Equation 11, is equal to the ratio of R to 0This output signal controls valve 35 to maintain the ratio at apreselected value.

In another embodiment Equation 11 can be expressed:

(l-l-KAT) R X (1+KAT) (12) where X is the square of the desired ratio ofinternal reflux to overhead vapor withdrawal rate. Equation 12 can beexpressed:

The term R is representative of a signal to be employed to control theexternal reflux to maintain X at a preselected value. In this embodimentcontroller 62 can comprise a computer and a flow controller. Thecomputer is provided with input signals representative of terms 0 X and(1+KAT) of Equation 13 and supplied an output signal R This outputsignal can be used to reset the flow controller which in turn regulatesvalve 35 in conduit 24. Signal X is supplied from an external source,which can be manually or automatically set, to provide the desiredratio. Where it can be assumed that the temperature of the overheadvapor remains substantially constant, the signal (1+KAT) can be based ona measurement of the temperature of the external reflux alone.

From the foregoing description it should be evident that improvedfractionation control systems are provided in accordance with thisinvention. These systems utilize indirect measurements of the internalreflux in the column. In response to such measurements, the ratio ofinternal reflux to feed or overhead vapor can be maintained atpreselected values. This substantially increases the stability of thecolumn by elimination or reduction of column upsets which are due tochanges in the temperature of reflux returned to the column.

While the invention has been described in conjunction with presentpreferred embodiments, it should be evident that it is not limitedthereto.

What is claimed is:

1. In a fractionation system wherein a fluid mixture to be separated isintroduced into a fractionation column through a first conduit, vaporsare removed from the top of said column through a second conduit, saidvapors are condensed and a portion of the condensate is returned to saidcolumn as reflux through a third conduit, and liquid is removed from thebottom of said column through a fourth conduit, a control systemcomprising first means operatively connected to said second conduit toestablish a first signal representative of the square of the rate offlow through said second conduit, second means operatively connected tosaid second and third conduits to establish a second signalrepresentative of the quantity (l-J-KAT) where K is a constant and AT isthe difference in temperature of fluids in said second and thirdconduits, third means operatively connected to said third conduit toestablish a third signal representative of the square of the rate offlow through said third conduit, fourth means responsive to said first,second and third means to establish a fourth signal representative ofthe product of said second and third signals divided by said firstsignal, and fifth means responsive to said fourth 15 means to adjust therate of flow through said third conduit.

2. The system of claim 1 further comprising means to maintain the howthrough said first conduit at a predetermined rate.

References Cited in the file of this patent UNITED STATES PATENTS1,735,470 Noel Nov. 1-2, 1929 2,236,035 Luhrs Mar. 25, 1941 2,459,404Anderson Jan. 18, 1949 2,589,651 Boyd Jan. 1, 1952 2,709,678 Berger May31, 1955 OTHER REFERENCES Instrument and Process Control, published byN.Y. State Vocational and Practical Arts Association, 1945 (pages155-185).

1. IN A FRACTIONATION SYSTEM WHEREIN A FLUID MIXTURE TO BE SEPARATED ISINTRODUCED INTO A FRACTIONATION COLUMN THROUGH A FIRST CONDUIT, VAPORSARE REMOVED FROM THE TOP OF SAID COLUMN THROUGH A SECOND CONDUIT, SAIDVAPORS ARE CONDENSED AND A PORTION OF THE CONDENSATE IS RETURNED TO SADCOLUMN AS REFLUX THROUGH A THIRD CONDUIT, AND LIQID IS REMOVED FROM THEBOTTOM OF SAID COLUMN THROUGH A FOURTH CONDUIT, A CONTROL SYSTEMCOMPRISING FIRST MEANS OPERATIVELY CONNECTED TO SAID SECOND CONDUIT TOESTABLISH A FIRST SINGAL REPRESENTATIVE OF THE SQUARE OF THE RATE OFFLOW THROUGH SAID SECOND CONDUIT, SECOND MEANS OPERATIVELY CONNECTED TOSAID SECOND AND THIRD CONDUITS TO ESTABLISH A SECOND SIGNALREPRESENTATIVE OF THE QUANTITY (1+5$T)2 WHERE K IS A CONSTANT AND $T ISTHE DIFFERENCE IN TEMPERATURE OF FLUIDS IN SAID SECOND AND THIRDCONDUITS, THIRD MEANS OPERATIVELY CONNECTED TO SAID THIRD CONDUIT TOESTABLISH A THIRD SIGNAL REPRESENTATIVE OF THE SQUARE OF THE RATE OFFLOW THROUGH SAID THIRD CONDUIT, FOURTH MEANS RESPONSIVE TO SAID FIRST,SECOND AND THIRD MEANS TO ESTABLISH OF FOURTH SIGNAL REPRESENTATIVE OFTHE PRODUCT OF SAID SECOND AND THIRD SIGNALS DIVIDED BY SAID FIRSTSIGNAL, AND FIFTH MEANS RESPONSIVE TO SAID FOURTH MEANS TO ADJUST THERATE OF FLOW THROUGH SAID THIRD CONDUIT.